Recently, there has been increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, generally have an average discharge voltage of about 3.6 to 3.7V, and drive voltage and energy density higher than those of conventional batteries (such as Ni-MH batteries, Ni—Cd batteries, etc.), and thus they are spotlighted in the field of secondary batteries.
However, as the fields where lithium secondary batteries are applied are gradually expanded, battery characteristics have been required to be improved from the standpoints of high-rate capability, cyclability, stability, etc. For example, high-rate capability refers to an indicator of the capability in high speed charge/discharge at high current, and is regarded as an important issue in the field requiring high power, such as a power tool, an HEV (Hybrid Electric Vehicle), etc. However, there is a problem in that lithium secondary batteries using a non-aqueous electrolyte have lithium ion conductivity lower than those of conventional batteries using an aqueous electrolyte (such as Ni-MH batteries, Ni—Cd batteries, etc.).
Also, a non-aqueous electrolyte used for secondary batteries usually includes an electrolyte solvent and an electrolyte salt. Herein the electrolyte solvent may be decomposed on the surface of an electrode during charge/discharge of a battery, or may break down an anode structure through co-intercalation between carbonaceous material anode layers, thereby obstructing the stability of the battery. It is known that such a problem can be solved by a solid electrolyte interface (hereinafter, referred to as SEI) layer formed on an anode surface by reduction of an electrolyte solvent during an initial charge of a battery, but in general, the SEI layer is insufficient to continuously protect an anode. Accordingly, the above mentioned problem may continuously occur during charge/discharge of a battery, thereby causing a problem in that the lifetime characteristic of the battery is degraded. Especially, when a battery is driven or left at high temperatures, the SEI layer is subject to break-down by increasing electrochemical and thermal energy with the passage of time due to its thermal instability. Accordingly, at high temperatures, there is a problem in that a battery characteristic is more significantly degraded, and particularly, gas (such as CO2) is continuously generated by the break-down of an SEI layer, the decomposition of an electrolyte, etc., thereby increasing the internal pressure and thickness of the battery.
In order to solve the above described problems, EU 683537 and JP 1996-45545 disclosed methods of using vinylene carbonate (hereinafter, referred to as VC) as an electrolyte additive for forming an SEI layer on an anode surface. However, the SEI layer formed by VC cannot improve high-rate capability and high temperature performance of a battery, because the SEI layer has a somewhat high resistance and is subject to break-down at high temperatures.
As described above, in a prior art, when a certain compound is added in an electrolyte in order to improve battery characteristics, some characteristics can be improved while other characteristics may be degraded or may be maintained as it is.